Since I have levels in Bard, can I copy (for example) mass cure wounds (a level 5 bard spell that isn't also a wizard spell) from a bard scroll into my spell book?
Could I do this idea with 3 levels in Cleric instead of in Bard?
The "Your Spellbook" sidebar states the following about copying spells into your spellbook (emphasis mine):
When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.
This passage disallows copying spells outside those available to a wizard into the spellbook, regardless of what other classes you've taken levels in.